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Old 23-09-2011, 20:56   #1
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Checking Battery AmpHours

Is it possible to check a batteries actual AH rather than what's printed on it? I have a large AGM and a Link 20 installed.

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Old 23-09-2011, 23:48   #2
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Re: Checking battery AH

The Ahr capacity of a battery depends on the discharge rate. The most meangful number is probably the AHr capacity at a normal discharge rate. To measure his fully charge the batteries , this is important.

Disconnect the charge sources.
Then use the link to measure the AHrs out until you hit 10.5v.

If you want to match the battery specs you do the same thing at a set discharge rate. 1/20 of the battery capacity (eg 10A for a 200Ahr battery) or whatever the battery is rated at.

Note the test will take the battery to completely flat, recharge it fully immediately afterwards and even then it will shorten the battery life. So you need a good reason to test them.

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Old 24-09-2011, 00:52   #3
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Re: Checking battery AH

Discharging a battery below 80% of it's capacity causes some damage to it's workings, depending on type.
These electric cars have one of the worst types of battery for damage incurred so a listed range of 100 miles is actually 20 miles if you don't want to replace the battery pack every 3 years.
Deep discharge batteries are a little better but mostly it is very difficult to get data on how long they will last (the downside of litigation for poor performance). Trip out devices at low voltage will protect the battery, but it's most likely to go at the end of a night passage, just as you need your instruments most. An override by-pass recommended.
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Old 24-09-2011, 12:04   #4
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Re: Checking Battery AmpHours

For a very thorough testing protocol see Section 4 at this link: Car and Deep Cycle Battery FAQ, Battery Manufacturers and Brand Names List, and

Taking an otherwise well maintained battery to 0% SOC one time will not adversely affect the long term battery performance in any measurable way, IF the battery is immediately and correctly recharge to 100% SOC.

That being said, unnecessary journeys below 50% SOC should be absolutely minimized to preclude a reduction in cycle life.

Charlie Johnson
JTB Marine Corporation
"The Devil is in the details and so is salvation."
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Old 24-09-2011, 14:25   #5
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Re: Checking Battery AmpHours

Since a battery when sold new has amp hrs listed, I wonder if your question is really about trying to determine what the amp hrs are in a used battery. All batteries degrade with use. Testing the individual cells for their specific gravity is what I use to keep track of how "healthy" my batteries are. The readings determine whether to equalize and give them a tune up if applicable (not all batteries qualify) or if eminent failure is on the horizon. I try not to pull out more than 20% of the amp hrs and I put it "all" back in every few days. This of course is in conflict with the 80% rule ( the last 20% takes the longest to recharge and is not considered economical) but making my rolls batteries last is important to me.
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